Molecular geochemistry of solid bitumen-bearing carbonate reservoir rocks from the Puguang Gas Field and nearby areas

Free and inclusion oils were recovered by sequential extraction from 15 solid bitumen-bearing carbonate reservoir rocks in the Puguang Gas Field and nearby areas,and then pyrolysates of the solid bitumens from these samples were obtained via confined pyrolysis experiments(gold capsules).All three components of each sample were further analyzed by GC and GC-MS for the biomarker compositions.The analytical results demonstrate that the ratio of gammacerane/C31 homohopanes is highest in solid bitumen pyrolysates,followed by free oils and inclusion oils.In addition,the molecular maturity parameters,such as ratios of C23 tricyclic terpane/(C23 tricyclic terpane + C30 hopane),C21/(C21 + ΣC29) steranes,C27 diasteranes/(C27 diasteranes + C27 regular steranes),20S/(20R+20S) C29 steranes and αββ/(ααα+αββ) C29 steranes of the inclusion oils are similar to those of the free oils,but substantially higher than those of the pyrolysates of solid bitumens.Based on various molecular parameters,it is suggested that the oils of paleo oil reservoirs probably had different sources.The early charged oils were derived from the Lower Devonian source rocks and had lower gammacerane contents and higher maturity,while the late charaged oils were derived from the Upper Permian source rocks and showed higher gammacerane contents and lower maturity.The solid bitumens were mainly generated from the cracking of the late charged oils.